feat: add Agrarmonitor correspondent sync

- Extract getOrCreateCorrespondent helper to deduplicate logic
- Add syncCorrespondentIds to match Paperless correspondents to
  Agrarmonitor IDs via Lieferantennummer and persist in CorrespondentSetting
- New POST /api/agrarmonitor/sync-correspondents endpoint
- "Agrarmonitor-Abgleich" button in Correspondents settings tab

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

paperless-frontend/src/api/settings.ts

@@ -225,4 +225,6 @@ export const agrarmonitorApi = {
     api.post<AgrarmonitorPollingResult>('/api/agrarmonitor/run-polling').then((r) => r.data),
   processUploads: () =>
     api.post<AgrarmonitorPollingResult>('/api/agrarmonitor/process-uploads').then((r) => r.data),
+  syncCorrespondents: () =>
+    api.post<{ total: number; matched: number; unmatched: number }>('/api/agrarmonitor/sync-correspondents').then((r) => r.data),
 };

paperless-frontend/src/pages/SettingsPage.tsx

@@ -1408,6 +1408,7 @@ function CorrespondentsTab() {
   const [pageSize, setPageSize] = useState(50);
   const [searchText, setSearchText] = useState('');
   const [createModalOpen, setCreateModalOpen] = useState(false);
+  const [syncLoading, setSyncLoading] = useState(false);
   const [form] = Form.useForm();
 
   const load = useCallback(async (page: number, size: number, search?: string) => {
@@ -1440,6 +1441,19 @@ function CorrespondentsTab() {
     }
   };
 
+  const handleSync = async () => {
+    setSyncLoading(true);
+    try {
+      const result = await agrarmonitorApi.syncCorrespondents();
+      message.success(`Abgleich abgeschlossen: ${result.matched} zugeordnet, ${result.unmatched} ohne Treffer (${result.total} gesamt)`);
+      load(currentPage, pageSize, searchText);
+    } catch (err) {
+      message.error('Fehler beim Agrarmonitor-Abgleich');
+    } finally {
+      setSyncLoading(false);
+    }
+  };
+
   const updateAgrarmonitorId = async (id: number, val: number | null) => {
     try {
       await settingsApi.updateCorrespondentSetting(id, val);
@@ -1497,9 +1511,14 @@ function CorrespondentsTab() {
             onSearch={(v) => { setSearchText(v); setCurrentPage(1); }}
             style={{ width: 300 }}
           />
-          <Button type="primary" icon={<PlusOutlined />} onClick={() => setCreateModalOpen(true)}>
-            Neuen Korrespondenten anlegen
-          </Button>
+          <Space>
+            <Button icon={<GlobalOutlined />} loading={syncLoading} onClick={handleSync}>
+              Agrarmonitor-Abgleich
+            </Button>
+            <Button type="primary" icon={<PlusOutlined />} onClick={() => setCreateModalOpen(true)}>
+              Neuen Korrespondenten anlegen
+            </Button>
+          </Space>
         </Space>
       </div>